In order for a GNSS receiver to calculate its position in relation to visible satellite vehicles, it must first receive information regarding how to interpret the PNT signals it receives from those satellites. This information is contained in navigation messages (conveyed as data bits) and may include: where satellite vehicles are currently/will be located (e.g., ephemeris or almanac data), current and predicted atmospheric conditions, satellite health, etc. A navigation message is typically embedded within PNT signals (i.e., modulated into the C/A code) and repeats in a cyclical manner. The information within the navigation message may receive periodic updates from a control segment which represent up-to-date conditions and observations. Typically, successive navigation messages are identical until such time that an update occurs.
When a receiver “cold-starts” (meaning, it begins attempting to acquire signals without any previously received navigation message data) it must initially “listen” to PNT signals from visible satellites to extract a navigation message. The navigation message will provide the receiver with information that will allow it to “lock-on” to the PNT signals. Only after locking on to the PNT signals is a receiver able to calculate its location and current time. In the case of an outdoor receiver, this process is routine. However, indoor receivers (for time-keeping, emergency services, etc.) may be unable to perceive the PNT signals with enough clarity to extract a navigation message. In other words, the strength of a radio signal may be too attenuated in an indoor environment for the receiver to comprehend a navigation message in the PNT signal due to insufficient sensitivity of the receiver. Without the navigation message, a receiver will likely be unable to acquire any satellite signals and will not function properly. Therefore, some receivers (e.g., assisted GNSS) acquire navigation messages through alternative channels to ensure the receiver has access to the information.
Some AGNSS receivers maintain a connection to a data network for reception of assistance data which includes navigation messages. For example, a receiver may have an internet connection, a mobile data network (e.g., 4G) antenna, etc. through which it receives navigation messages. A receiver may utilize this information in the generation of a replica signal which it then uses to assess correlation with regard to a received PNT signal in order to perform calculations such as, for example, a pseudo-range to a satellite vehicle which is in turn used to calculate time and position. Theoretically, given that an assisted receiver knows the navigation message data that will be embedded in the C/A signal, the replica signal should be identical to the signal transmitted from the respective satellite vehicle and correlation should be accurate. However, in the absence of assistance data (e.g., due to a network outage), the replica signal is likely to contain errors because certain bits of information may have been updated in a more recent navigation message while the information on the receiver remains outdated. Therefore, in the event that a receiver's data connection is lost, it may “lose” a PNT signal as a result of correlation degradation. In this regard, a solution is needed to address the problem of receivers dropping signals when assistance data cannot be received through alternative channels.